This invention relates generally to golf balls, and, more specifically, to a two-piece golf ball. In particular, this invention relates to a two-piece golf ball having a core and an outer cover layer, wherein the core composition comprises at least one peptizer. The two-piece golf balls of the present invention provide an increase in driver ball speed, or the ball's coefficient of restitution (“C.O.R.”), as compared to balls having an identical core and outer cover layer, but lacking a peptizer.
Until relatively recently, many highly skilled, low-handicap golfers had utilized wound golf balls for competitive play. These balls have a wound core (i.e., a liquid or solid center and elastic windings) and a balata or balata-like cover.
Balata-covered wound golf balls are known for their soft feel and high spin rate potential. However, balata-covered balls suffer from the drawback of low durability. Among other things, even in normal use, the balata cover can become cut and scuffed, making the ball unsuitable for further play.
For golf ball cover materials, balata was generally the industry standard until about the middle 1960's, when E.I. DuPont de Nemours & Co. introduced a new series of resins, now known as ionomers (see U.S. Pat. No. 3,264,272, (“the '272 patent”)). These ionomeric resins have been found to function quite well as golf ball cover materials (see U.S. Pat. No. 3,454,280). The ionomeric resins are sold by DuPont under the trademark Surlyn®. Although the '272 patent identified above teaches a wide variety of compounds, only a relatively few resins are available commercially. The ionomeric resins are copolymers of an olefin and an α,β-ethylenically unsaturated carboxylic acid, with 5-100% of the carboxylic acid groups being neutralized by metal ions.
Although balata is still used to a considerable extent, the ionomeric resins have dominated the golf ball cover market and have replaced the balata covers except for particularly exceptional golfers and those who seek to emulate them. Reasons for the success of these ionomeric resins include their improved cut resistance, as compared to balata, their substantially cheaper price, and their ready availability. In connection with the latter reason, it is pointed out that balata is a natural resin imported from foreign countries and its availability, at best, is erratic, and prices fluctuate accordingly.
When making golf ball covers, the Surlyn® resins can be used individually or they can be blended together. U.S. Pat. No. 3,819,768 teaches that a blend of a sodium salt of an ionomer resin with a zinc salt of an ionomer resin will provide improved cold-cracking properties.
Until the late 1960's, a balata cover was typically used with a so-called wound core, comprising a center and a long, thin elastic band wrapped tightly thereabout. The internal elastic windings are susceptible to breakage upon ball striking, and the center of the ball can be knocked off center or “out of round.” Furthermore, the coefficient of restitution of wound balls is reduced at low temperatures.
In the late 1960's, compositions were developed that could replace the wound cores. It was originally thought that these materials could replace both the core and the cover, thus enabling the production of a unitary, homogeneous golf ball. However, the unitary golf balls have not been found to be of sufficiently high quality for play by most golfers. Unitary golf balls are played by some golfers, and a large number of such balls are used as practice balls in driving ranges and the like. Nevertheless, the consensus of opinion today is that a high-grade golf ball should include a unitary solid core of approximately the dimensions of a wound core, together with a cover thereon.
The golf industry has generally settled on one type of core composition for use in solid golf balls to achieve the best properties. This composition is polybutadiene with a high cis-1,4-content, cross-linked using peroxide or sulfur compounds, a zinc dimethacrylate or zinc diacrylate co-cross-linking agent, and a filler such as zinc oxide. Of the two co-cross-linkers, zinc diacrylate, has been found to produce golf balls having greater initial velocity than zinc dimethacrylate.
These core compositions provide improved properties; however, even with the blending of materials, ideal properties have not been achieved in golf balls. For example, a high C.O.R. is preferred because, as mentioned above, it correlates with greater ball flight distance. An increased loading level of sulfur compounds, peroxides, or acrylate metal salts in the polybutadiene rubber used for a core composition is known to increase C.O.R. However, this also will lead to increased core compression, resulting in poorer ball feel. This relationship between C.O.R. and compression can be adjusted only to a limited extent using known activators, cross-linking agents, and co-cross-linking agents.
A variety of other materials, such as fillers and processing aids, also have been used in making golf balls cores. For example, small amounts of chemical peptizers have been incorporated in golf ball cores. These peptizers generally are used in small amounts to retard the initial curing of polybutadiene rubber under the influence of mechanical force, heat, or a combination of these. Use of these peptizers allows for incorporation of a wider array of active chemical ingredients and fillers in a composition. These peptizers also provide for easier processing of these compositions, as well as lower processing temperatures.
For example, U.S. Pat. No. 5,252,652 discloses that adding an organic sulfur compound and/or a metal-containing organic sulfur compound to the rubber composition used to prepare the core of a one-piece or a multi-layered golf ball, the resulting ball exhibits an increased initial driver velocity and improved flying performance. The rubber composition contains an organic sulfur compound and/or a metal-containing organic sulfur compound in addition to the base rubber, unsaturated carboxylic acid metal salt, and organic peroxide.
U.S. Pat. No. 5,776,012 discloses a solid golf ball comprising a core and a cover, wherein the core is formed from a vulcanized molded material of a rubber composition comprising a base rubber, a metal salt of an unsaturated carboxylic acid, an organic peroxide, an organic sulfur compound and/or a metal-containing organic sulfur compound, and a filler. The organic sulfur compound and metal-containing organic sulfur compound are said to accelerate mastication of the rubber, thereby enhancing the rebound performance of the soft part of the core. The organic sulfur compound can be diphenyl disulfide, dixylyl disulfide, pentachlorothiophenol, thiobenzoic acid, morpholine disulfide, etc. The metal-containing organic sulfur compound can be a zinc salt of thiophenols or thiocarboxylic acids.
U.S. Patent Publication No. 2003/0064826A1 and U.S. Pat. No. 6,635,716 disclose that adding a halogenated organic sulfur compound or the salts thereof, in particular, pentachlorothiophenol (“PCTP”) salt, to polybutadiene rubber compositions used in golf ball cores provides increased C.O.R., decreased compression, or both. The polybutadiene rubber composition may include between about 2.2 parts and about 5 parts of a halogenated organic sulfur compound, or the metal salt thereof. The metal salt may be zinc, calcium, potassium, magnesium, sodium, or lithium, but is preferably zinc.
The United States Golf Association (“USGA”) has set certain standards for golf balls. The initial velocity of the golf ball is set at a maximum of 255 feet per second (250 feet per second with a 2% tolerance), when measured using USGA standards. Golf ball manufacturers strive to come as close to this limit as possible, without exceeding it. However, it has proven difficult for solid golf balls to reach this limit. An improvement of only one-half foot per second is considered significant.
It therefore will be appreciated that there is a need for an improved two-piece golf ball having an optimal velocity and distance performance and excellent shot feel, the so-called “long and soft” combination, while also providing excellent durability properties and ease of manufacture. The present invention fulfills this need and provides further related advantages.